Polypropylene compositions are widely used in the plastics industry. Polypropylene compositions can be processed to form molded bodies with unique shapes and advantageous mechanical properties including high hardness, stiffness, and stability. Consumer articles including automotive parts, toys, containers, or household goods are often produced. However, such formulations are often excluded from applications with more demanding specifications due to inadequate properties—most notably poor low-temperature impact. Propylene compositions with a) high stiffness b) good clarity c) resistance to stress whitening and d) excellent low temperature impact properties are required. This is difficult to achieve because enhancement of one property generally comes at the expense of another property.
In the art, highly crystalline polypropylene is generally used to attain very stiff and tough materials with excellent mechanical properties. However, high crystallinity significantly increases the haze and worsens optical properties. Addition of higher amounts of comonomer would mean reduced crystallinity and lower haze but also decreased stiffness. The balance of components must be very carefully planned to get a desired result.
It is also well known in the art that the addition of an elastomeric phase to a polypropylene matrix will improve the impact properties of the composition. However, the types of rubber that may be used without affecting the favorable properties of the matrix are limited. While incorporating an elastomer will improve impact properties, stress whitening and opacity will often worsen.
U.S. Pat. No. 4,734,459 discloses an inventive compound of polypropylene elastomer material with good stress whitening resistance. EP 0,730, 003 shows that whitening resistance in U.S. Pat. No. 4,734,459 can be further improved by changing an ethylene-propylene rubber to an ethylene-butene rubber, which reinforces the importance of elastomer selection.
EP 2,340,279 and EP 2,471, 858 disclose compositions of polypropylene blends that are resistant against stress whitening and exhibit improved clarity in articles molded therefrom.
Patents EP 1,730,231, EP 1,702,956, and U.S. Pat. No. 8,173,747 each disclose inventive compounds with excellent transparency, good stress whitening, and improved impact properties. Molded articles produced from these compositions are used in applications requiring thin, flexible films. However, materials with high stiffness remain elusive.
U.S. Pat. No. 7,476,710 describes a material possessing the closest properties to the elusive balance of stiffness, whitening, impact, and clarity, which are required for demanding applications. U.S. Pat. No. 7,476,710 describes a molded article comprising a heterogeneous blend comprising: (1) from 60 to 99 weight percent of one or more semi-crystalline polymers (based upon the weight of the semi-crystalline and semi-amorphous polymers), each semi-crystalline polymer comprising propylene and from 0 to 5 weight % alpha-olefin comonomer (based upon the weight of the polymer), said semi-crystalline polymers each having a melting point between 100 and 170° C. and a melt flow rate of 200 dg/min or less (preferably 1 to 100 dg/min or less); and (2) from 1 to 40 weight % of one or more semi-amorphous polymers (based upon the weight of the semi-crystalline and semi-amorphous polymers), each semi-amorphous polymer comprising propylene and from 10 to 25 weight % of one or more C2 and/or C4 to C10 alpha-olefin comonomers, said semi-amorphous polymers each having:                a) a heat of fusion of 4 to 70 J/g; and        b) a melt flow rate of 0.1 to 200 dg/min (preferably 1 to 100 dg/min or less); and        c) an intermolecular compositional distribution as determined by thermal fractionation in hexane such that 85% by weight or more of the polymer is isolated as one or two adjacent, soluble fractions with the balance of the polymer in immediately preceding or succeeding fractions; and wherein each of these fractions has a wt % comonomer content with a difference of no greater than 20 wt % relative to the average wt % comonomer content of the copolymer;        d) an Mw/Mn of 1.5 to 4, and        e) a propylene triad tacticity, as measured by 13C NMR, of 75% or greater, where the blend has:                    i) a melt flow rate of 0.5 to 200 dg/min (preferably 1 to 100 dg/min); and            ii) less than 5 weight % filler, based upon the weight of the polymers and the filler; and            iii) a permanent set of greater than 65%; and            iv) a haze of 20% or less on a 1 mm injection molded chip; and the molded part has:                            a) a thickness of 250 μm to 10 mm; and                b) a notched Izod impact strength at 23° C. of 80 J/m (1.5 ft·lb/in) or more; and                c) a notched Izod impact strength at 0° C. of 27 J/m (0.5 ft·lb/in) or more; and                d) a 1% secant flexural modulus of 150,000 psi (1035 MPa) or lower                e) an ultimate elongation where greater than 50% of 5 test specimens suffer no break through extension to 1000%; and                f) a resistance to stress whitening of Hunter color ΔL of 15 or lower, measured on a 0.125 in (3.18 mm) thick injection molded pad.                                                
The properties of the molded articles described above are an improvement to its predecessors. However, an even better balance of impact, stress whitening resistance, haze, and strength must be achieved to expand polypropylene use into the most demanding plastic applications. Most importantly, molded parts should also have low temperature and sub-ambient impact strength and ductility. The current invention fulfills and exceeds these impact requirements, while maintaining a balance of stress whitening, haze, and strength.